when do i replace my grinding wheel?

[croftie]

Hi, I am a newbie here. I have had a T3 for many years and have found it absolutely essential for chisels, planes, planer knives and also domestic cooking knives for my partner, family and friends. As a result my stone has worn down such that when i grind longer knives it now clashes with the honing wheel. So I am now wondering is it time to replace the wheel? I am asking because the cost at £75 is a lot although i know i have had excellent value from that old stone! Any thoughts?

[Rhino]

I am sure  somebody will tell you the official line.  I don't know.

But don't feel bad. Based on the geometry, most of the grinding ability, most of the stone, is on the outside.  If you used up the outside, you have used up most of it.

[Herman Trivilino]

A good way to look at the math is to think about the relationship between the diameter of the grindstone and its area.  The area is proportional to the square of the diameter.  So, for example, if you have 50% of the diameter left, you have only 25% of the area left (because 0.50² = 0.25).  You've used up 75% of the grindstone!  And the remaining 25% will wear away all the faster.

I'm not familiar with the T3, but on the T7 if the honing wheel is in the way you can easily remove it.  The real issue is the concave grind you'll get.  You'll notice that the bevel on your chisels, for example, has a concave surface.  This may affect their performance.  If you can put up with that, keep using that grindstone.